SONAR (SOund Navigation And Ranging)—or sonar—is the generic name of the technology that is used to locate objects underwater. Sonar is used in marine, geological, and biological research, undersea mapping and navigation, and various commercial and military applications.
An “active” sonar system is a type of sonar system in which a “projector” emits a pulse of sound and underwater microphones called “hydrophones” receive underwater sounds to be signal processed. If the transmitted pulses encounter an underwater object (a “target”), a portion of the sound is reflected as an “echo.” Knowing the speed of sound in water and the time for the sound wave to travel to the target and back, the distance between the listening-post vessel (e.g., ship, etc.) and the target can be calculated. Active sonar systems generally use highly directional beams of sound when searching for targets, which enable them to determine direction to the target, as well as the distance.
Another application of active sonar processing is for measuring the velocity of the sound-projecting vessel itself. The sonar source of the vessel directs sonic pulses towards the ocean floor, and the receivers detect echoes of those pulses. The velocity of the vessel is then calculated based upon the distance traveled by the vessel between the transmission and reception of a first pulse and a second pulse. Examples of velocity-measurement sonars are spatial correlation sonar and temporal correlation sonar, which rely on selecting a maximum “correlation” between hydrophones in the case of spatial correlation or pulses in the case of temporal correlation.
A spatial correlation sonar requires an array of many hydrophones with associated electronics components; consequently, spatial correlation sonar might be burdened with major ship's array installation costs. In contrast, a basic temporal correlation sonar requires only three hydrophones with reduced support electronics; as a result, temporal correlation sonar is likely to have lower ship's installation costs. A spatial system requires substantial processing to form arrays and extract ship's velocity via time-consuming array processing, whereas processor throughput requirements are reduced for a temporal system which does not need to deal with multiple velocity vectors and time-consuming array processing.
With respect to operating conditions, a spatial correlation system performs well for low ship's speeds, but its performance steadily degrades as speed increases. This is because it must reduce the time between correlated pulses (i.e., the correlation time) in order to keep the correlation solution limited to involving hydrophone pairs inside the limited-size hydrophone array while allowing for ship's dynamics. A spatial system's correlation time is a compromise value to allow for (i) a maximum ship's speed in one chosen direction (e.g., forward), (ii) the use of as large a value as possible consistent with the hydrophone array dimensions, and (iii) the use of a value that allows for variations in ship's velocity estimates in a sonar cycle. These conditions may result in a spatial correlation sonar being unable to make use of the full hydrophone array size.
In contrast, a temporal correlation system cannot function for low ship's speeds because this requires the use of an excessively long and often unachievable time between correlated pulses (i.e., the time separation between correlated pulses which exceeds the transmit-to-echo roundtrip time). A temporal correlation sonar, however, provides a performance enhancement for high ship's speeds by making full use of the size of a hydrophone array in all directions and permits further high speed performance improvements by use of available hydrophones that are external to a correlation sonar hydrophone array.
What is needed is an integrated sonar system that provides the performance benefits of both spatial and temporal correlation sonars, as well as a means of detecting whether one of these correlation sonar systems is performing poorly, without some of the disadvantages in the prior art.